This project involves the examination of mammalian cells that display a G1 interval (G1 plus cells) and cells that lack G1 (G1-). The G1-phenotype has been found to behave dominantly in intraspecific C. hamster hybrids. G1 mutants of a G1- line defining at least four complementation groups (complementation is exemplified by two G1 plus cells forming G1- hybrids) have been isolated. Slowing protein synthesis in the G1- parent with cycloheximide results in the appearance of G1 without affecting S, G2, or M. Three of four mutants examined appear to affect protein synthesis, hence their G1 intervals. One mutant does not affect protein synthesis or degradation. Non-mutant G1 plus hamster lines and strains have been fused and define at least two G1 plus complementation groups (e.g., G1 plus CHO x G1 plus CHIII yields G1- hybrids). These results suggest that different G1 plus cells can have different underlying causes for their G1 intervals. At present, work designed to look at the G1 plus phenotypes of differentiated cell types is being performed.